This invention relates to electrical waveform synthesis using switching circuits and in particular but not solely switching circuit waveform synthesisers for use as AC motor drives.
An alternating current waveform can be derived or synthesised from a direct current by appropriately switching the DC source. Equipment for producing an AC waveform from a DC source are known as inverters. These inverters may be either static or non-static. Static inverters are much preferred and may be formed using suitable solid state devices such as thyristors or triacs. Whether the inverter be static or non-static the output is a complex waveform, whereas it is frequently desirable that the output be sinusoidal. Sinusoidal waveforms can be produced be suppressing unwanted harmonics contained in the switching circuit output waveform. For example, a low pass or integrating filter can be used to pass only the lower frequency or fundamental waveform which is generally a sinusoid. In some instances the electric circuit or load into which the waveform is injected acts as its own integrating filter. Such is the case with the AC electric motor. Such a motor can be driven from a simple inverter producing a square wave output. If, as would be expected to be the case, the motor was 3-phase then by the use of a 3-phase inverter the line voltage will have reduced low order harmonics even though the phase voltages are square waves. However, it is desirable to drive such motors by an AC waveform more closely approximating a sinusoid.
To allow for complete control over the inverter output waveform characteristics it is usual to use an external pulse source to switch the inverter. The inverter switching pulses are produced by low power circuitry and the output from this circuitry (which would normally be a unidirectional voltage waveform) is used to switch a static inverter per se which in turn switches the power to the load (as a bi-directional voltage waveform). The pulse source circuitry may be conveniently formed using integrated circuits. It is relatively easy using such techniques to synthesise a complex waveform having a variable fundamental frequency which may then be used to appropriately switch an inverter. This feature is particularly significant since the availability of high power variable frequency AC means that AC motors can be used in variable speed applications. In most situations the mains supply is AC and, accordingly, variable speed AC motor drives must normally include associated rectifier and smoothing equipment to produce the inverter input DC. This equipment is however already required for DC motor drives which at present predominate. A variable speed application where the use of an AC motor would be particularly attractive, and incidently where the mains supply is DC, is as a power source for vehicles.
A requirement of an AC motor is that if it is desired to vary motor speed by varying the supply frequency, the supply voltage should also be varied proportionally to ensure a constant voltage-time product and thus constant flux. A known method for satisfying this requirement is to vary the voltage of the inverter DC supply by the use of a chopper. For reasons of simplicity and economics it would be desirable to control the inverter output amplitude by appropriately controlling the low power switching pulses.
A known technique for synthesising a low frequency sinusoid using two state switching circuits is to generate a pulse width modulated waveform, the width of the pulses being related to the amplitude of the desired sinusoid at any instant. This pulse width modulated waveform is then passed through an integrating filter designed to pass only the desired sinusoid. A common method of obtaining the necessary pulse width modulated waveform is to compare a reference sinusoid with a high frequency carrier having a trianglular or saw-toothed waveform. This method requires complex linear and digital circuitry, particularly for 3-phase systems which require both phase balance and voltage amplitude control over wide frequency and amplitude ranges. It would be desirable for such a synthesiser to utilise purely digital circuitry especially as the derivation of a 3-phase sinusoidal signal is thereby simplified.